man2/semctl.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\"
  24 .\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esr@thyrsus.com>
  25 .\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aeb@cwi.nl>
  26 .\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aeb@cwi.nl>
  27 .\" Modified 20 Dec 2001, Michael Kerrisk <mtk.manpages@gmail.com>
  28 .\" Modified 21 Dec 2001, aeb
  29 .\" Modified 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  30 .\"     Added notes on CAP_IPC_OWNER requirement
  31 .\" Modified 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  32 .\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
  33 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  34 .\"     Language and formatting clean-ups
  35 .\"     Rewrote semun text
  36 .\"     Added semid_ds and ipc_perm structure definitions
  37 .\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions.
  38 .\"
  39 .TH SEMCTL 2 2008-05-20 "Linux" "Linux Programmer's Manual"
  40 .SH NAME
  41 semctl \- semaphore control operations
  42 .SH SYNOPSIS
  43 .nf
  44 .B #include <sys/types.h>
  45 .B #include <sys/ipc.h>
  46 .B #include <sys/sem.h>
  47 .sp
  48 .BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"
  49 .fi
  50 .SH DESCRIPTION
  51 .BR semctl ()
  52 performs the control operation specified by
  53 .I cmd
  54 on the semaphore set identified by
  55 .IR semid ,
  56 or on the
  57 .IR semnum -th
  58 semaphore of that set.
  59 (The semaphores in a set are numbered starting at 0.)
  60 .PP
  61 This function has three or four arguments, depending on
  62 .IR cmd .
  63 When there are four, the fourth has the type
  64 .IR "union semun" .
  65 The \fIcalling program\fP must define this union as follows:
  66
  67 .nf
  68 .in +4n
  69 union semun {
  70     int              val;    /* Value for SETVAL */
  71     struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */
  72     unsigned short  *array;  /* Array for GETALL, SETALL */
  73     struct seminfo  *__buf;  /* Buffer for IPC_INFO
  74                                 (Linux-specific) */
  75 };
  76 .in
  77 .fi
  78 .PP
  79 The
  80 .I semid_ds
  81 data structure is defined in \fI<sys/sem.h>\fP as follows:
  82 .nf
  83 .in +4n
  84
  85 struct semid_ds {
  86     struct ipc_perm sem_perm;  /* Ownership and permissions */
  87     time_t          sem_otime; /* Last semop time */
  88     time_t          sem_ctime; /* Last change time */
  89     unsigned short  sem_nsems; /* No. of semaphores in set */
  90 };
  91 .in
  92 .fi
  93 .PP
  94 The
  95 .I ipc_perm
  96 structure is defined in \fI<sys/ipc.h>\fP as follows
  97 (the highlighted fields are settable using
  98 .BR IPC_SET ):
  99 .PP
 100 .nf
 101 .in +4n
 102 struct ipc_perm {
 103     key_t          key;   /* Key supplied to semget(2) */
 104     uid_t          \fBuid\fP;   /* Effective UID of owner */
 105     gid_t          \fBgid\fP;   /* Effective GID of owner */
 106     uid_t          cuid;  /* Effective UID of creator */
 107     gid_t          cgid;  /* Effective GID of creator */
 108     unsigned short \fBmode\fP;  /* Permissions */
 109     unsigned short seq;   /* Sequence number */
 110 };
 111 .in
 112 .fi
 113 .PP
 114 Valid values for
 115 .I cmd
 116 are:
 117 .TP 12
 118 .B IPC_STAT
 119 Copy information from the kernel data structure associated with
 120 .I semid
 121 into the
 122 .I semid_ds
 123 structure pointed to by
 124 .IR arg.buf .
 125 The argument
 126 .I semnum
 127 is ignored.
 128 The calling process must have read permission on the semaphore set.
 129 .TP
 130 .B IPC_SET
 131 Write the values of some members of the
 132 .I semid_ds
 133 structure pointed to by
 134 .I arg.buf
 135 to the kernel data structure associated with this semaphore set,
 136 updating also its
 137 .I sem_ctime
 138 member.
 139 The following members of the structure are updated:
 140 .IR sem_perm.uid ,
 141 .IR sem_perm.gid ,
 142 and (the least significant 9 bits of)
 143 .IR sem_perm.mode .
 144 The effective UID of the calling process must match the owner
 145 .RI ( sem_perm.uid )
 146 or creator
 147 .RI ( sem_perm.cuid )
 148 of the semaphore set, or the caller must be privileged.
 149 The argument
 150 .I semnum
 151 is ignored.
 152 .TP
 153 .B IPC_RMID
 154 Immediately remove the semaphore set,
 155 awakening all processes blocked in
 156 .BR semop (2)
 157 calls on the set (with an error return and
 158 .I errno
 159 set to
 160 .BR EIDRM ).
 161 The effective user ID of the calling process must
 162 match the creator or owner of the semaphore set,
 163 or the caller must be privileged.
 164 The argument
 165 .I semnum
 166 is ignored.
 167 .TP
 168 .BR IPC_INFO " (Linux-specific)"
 169 Returns information about system-wide semaphore limits and
 170 parameters in the structure pointed to by
 171 .IR arg.__buf .
 172 This structure is of type
 173 .IR seminfo ,
 174 defined in
 175 .I <sys/sem.h>
 176 if the
 177 .B _GNU_SOURCE
 178 feature test macro is defined:
 179 .nf
 180 .in +4n
 181
 182 struct  seminfo {
 183     int semmap;  /* Number of entries in semaphore
 184                     map; unused within kernel */
 185     int semmni;  /* Maximum number of semaphore sets */
 186     int semmns;  /* Maximum number of semaphores in all
 187                     semaphore sets */
 188     int semmnu;  /* System-wide maximum number of undo
 189                     structures; unused within kernel */
 190     int semmsl;  /* Maximum number of semaphores in a
 191                     set */
 192     int semopm;  /* Maximum number of operations for
 193                     semop(2) */
 194     int semume;  /* Maximum number of undo entries per
 195                     process; unused within kernel */
 196     int semusz;  /* Size of struct sem_undo */
 197     int semvmx;  /* Maximum semaphore value */
 198     int semaem;  /* Max. value that can be recorded for
 199                     semaphore adjustment (SEM_UNDO) */
 200 };
 201
 202 .in
 203 .fi
 204 The
 205 .IR semmsl ,
 206 .IR semmns ,
 207 .IR semopm ,
 208 and
 209 .I semmni
 210 settings can be changed via
 211 .IR /proc/sys/kernel/sem ;
 212 see
 213 .BR proc (5)
 214 for details.
 215 .TP
 216 .BR SEM_INFO " (Linux-specific)"
 217 Returns a
 218 .I seminfo
 219 structure containing the same information as for
 220 .BR IPC_INFO ,
 221 except that the following fields are returned with information
 222 about system resources consumed by semaphores: the
 223 .I semusz
 224 field returns the number of semaphore sets that currently exist
 225 on the system; and the
 226 .I semaem
 227 field returns the total number of semaphores in all semaphore sets
 228 on the system.
 229 .TP
 230 .BR SEM_STAT " (Linux-specific)"
 231 Returns a
 232 .I semid_ds
 233 structure as for
 234 .BR IPC_STAT .
 235 However, the
 236 .I semid
 237 argument is not a semaphore identifier, but instead an index into
 238 the kernel's internal array that maintains information about
 239 all semaphore sets on the system.
 240 .TP
 241 .B GETALL
 242 Return
 243 .B semval
 244 (i.e., the current value)
 245 for all semaphores of the set into
 246 .IR arg.array .
 247 The argument
 248 .I semnum
 249 is ignored.
 250 The calling process must have read permission on the semaphore set.
 251 .TP
 252 .B GETNCNT
 253 The system call returns the value of
 254 .B semncnt
 255 (i.e., the number of processes waiting for the value of
 256 this semaphore to increase) for the
 257 .IR semnum \-th
 258 semaphore of the set
 259 (i.e., the number of processes waiting for an increase of
 260 .B semval
 261 for the
 262 .IR semnum \-th
 263 semaphore of the set).
 264 The calling process must have read permission on the semaphore set.
 265 .TP
 266 .B GETPID
 267 The system call returns the value of
 268 .B sempid
 269 for the
 270 .IR semnum \-th
 271 semaphore of the set
 272 (i.e. the PID of the process that executed the last
 273 .BR semop (2)
 274 call for the
 275 .IR semnum \-th
 276 semaphore of the set).
 277 The calling process must have read permission on the semaphore set.
 278 .TP
 279 .B GETVAL
 280 The system call returns the value of
 281 .B semval
 282 for the
 283 .IR semnum \-th
 284 semaphore of the set.
 285 The calling process must have read permission on the semaphore set.
 286 .TP
 287 .B GETZCNT
 288 The system call returns the value of
 289 .B semzcnt
 290 (i.e., the number of processes waiting for the value of
 291 this semaphore to become zero) for the
 292 .IR semnum \-th
 293 semaphore of the set
 294 (i.e. the number of processes waiting for
 295 .B semval
 296 of the
 297 .IR semnum \-th
 298 semaphore of the set to become 0).
 299 The calling process must have read permission on the semaphore set.
 300 .TP
 301 .B SETALL
 302 Set
 303 .B semval
 304 for all semaphores of the set using
 305 .IR arg.array ,
 306 updating also the
 307 .I sem_ctime
 308 member of the
 309 .I semid_ds
 310 structure associated with the set.
 311 Undo entries (see
 312 .BR semop (2))
 313 are cleared for altered semaphores in all processes.
 314 If the changes to semaphore values would permit blocked
 315 .BR semop (2)
 316 calls in other processes to proceed, then those processes are woken up.
 317 The argument
 318 .I semnum
 319 is ignored.
 320 The calling process must have alter (write) permission on
 321 the semaphore set.
 322 .TP
 323 .B SETVAL
 324 Set the value of
 325 .B semval
 326 to
 327 .I arg.val
 328 for the
 329 .IR semnum \-th
 330 semaphore of the set, updating also the
 331 .I sem_ctime
 332 member of the
 333 .I semid_ds
 334 structure associated with the set.
 335 Undo entries are cleared for altered semaphores in all processes.
 336 If the changes to semaphore values would permit blocked
 337 .BR semop (2)
 338 calls in other processes to proceed, then those processes are woken up.
 339 The calling process must have alter permission on the semaphore set.
 340 .SH "RETURN VALUE"
 341 On failure
 342 .BR semctl ()
 343 returns \-1
 344 with
 345 .I errno
 346 indicating the error.
 347
 348 Otherwise the system call returns a non-negative value depending on
 349 .I cmd
 350 as follows:
 351 .TP 11
 352 .B GETNCNT
 353 the value of
 354 .BR semncnt .
 355 .TP
 356 .B GETPID
 357 the value of
 358 .BR sempid .
 359 .TP
 360 .B GETVAL
 361 the value of
 362 .BR semval .
 363 .TP
 364 .B GETZCNT
 365 the value of
 366 .BR semzcnt .
 367 .TP
 368 .B IPC_INFO
 369 the index of the highest used entry in the
 370 kernel's internal array recording information about all
 371 semaphore sets.
 372 (This information can be used with repeated
 373 .B SEM_STAT
 374 operations to obtain information about all semaphore sets on the system.)
 375 .TP
 376 .B SEM_INFO
 377 As for
 378 .BR IPC_INFO .
 379 .TP
 380 .B SEM_STAT
 381 the identifier of the semaphore set whose index was given in
 382 .IR semid .
 383 .LP
 384 All other
 385 .I cmd
 386 values return 0 on success.
 387 .SH ERRORS
 388 On failure,
 389 .I errno
 390 will be set to one of the following:
 391 .TP 11
 392 .B EACCES
 393 The argument
 394 .I cmd
 395 has one of the values
 396 .BR GETALL ,
 397 .BR GETPID ,
 398 .BR GETVAL ,
 399 .BR GETNCNT ,
 400 .BR GETZCNT ,
 401 .BR IPC_STAT ,
 402 .BR SEM_STAT ,
 403 .BR SETALL ,
 404 or
 405 .B SETVAL
 406 and the calling process does not have the required
 407 permissions on the semaphore set and does not have the
 408 .B CAP_IPC_OWNER
 409 capability.
 410 .TP
 411 .B EFAULT
 412 The address pointed to by
 413 .I arg.buf
 414 or
 415 .I arg.array
 416 isn't accessible.
 417 .TP
 418 .B EIDRM
 419 The semaphore set was removed.
 420 .TP
 421 .B EINVAL
 422 Invalid value for
 423 .I cmd
 424 or
 425 .IR semid .
 426 Or: for a
 427 .B SEM_STAT
 428 operation, the index value specified in
 429 .I semid
 430 referred to an array slot that is currently unused.
 431 .TP
 432 .B EPERM
 433 The argument
 434 .I cmd
 435 has the value
 436 .B IPC_SET
 437 or
 438 .B IPC_RMID
 439 but the effective user ID of the calling process is not the creator
 440 (as found in
 441 .IR sem_perm.cuid )
 442 or the owner
 443 (as found in
 444 .IR sem_perm.uid )
 445 of the semaphore set,
 446 and the process does not have the
 447 .B CAP_SYS_ADMIN
 448 capability.
 449 .TP
 450 .B ERANGE
 451 The argument
 452 .I cmd
 453 has the value
 454 .B SETALL
 455 or
 456 .B SETVAL
 457 and the value to which
 458 .B semval
 459 is to be set (for some semaphore of the set) is less than 0
 460 or greater than the implementation limit
 461 .BR SEMVMX .
 462 .SH "CONFORMING TO"
 463 SVr4, POSIX.1-2001.
 464 .\" SVr4 documents more error conditions EINVAL and EOVERFLOW.
 465 .SH NOTES
 466 The
 467 .BR IPC_INFO ,
 468 .B SEM_STAT
 469 and
 470 .B SEM_INFO
 471 operations are used by the
 472 .BR ipcs (8)
 473 program to provide information on allocated resources.
 474 In the future these may modified or moved to a /proc file system
 475 interface.
 476 .LP
 477 Various fields in a \fIstruct semid_ds\fP were typed as
 478 .I short
 479 under Linux 2.2
 480 and have become
 481 .I long
 482 under Linux 2.4.
 483 To take advantage of this,
 484 a recompilation under glibc-2.1.91 or later should suffice.
 485 (The kernel distinguishes old and new calls by an
 486 .B IPC_64
 487 flag in
 488 .IR cmd .)
 489 .PP
 490 In some earlier versions of glibc, the
 491 .I semun
 492 union was defined in \fI<sys/sem.h>\fP, but POSIX.1-2001 requires
 493 that the caller define this union.
 494 On versions of glibc where this union is \fInot\fP defined,
 495 the macro
 496 .B _SEM_SEMUN_UNDEFINED
 497 is defined in \fI<sys/sem.h>\fP.
 498 .PP
 499 The following system limit on semaphore sets affects a
 500 .BR semctl ()
 501 call:
 502 .TP 11
 503 .B SEMVMX
 504 Maximum value for
 505 .BR semval :
 506 implementation dependent (32767).
 507 .LP
 508 For greater portability it is best to always call
 509 .BR semctl ()
 510 with four arguments.
 511 .SH "SEE ALSO"
 512 .BR ipc (2),
 513 .BR semget (2),
 514 .BR semop (2),
 515 .BR capabilities (7),
 516 .BR sem_overview (7),
 517 .BR svipc (7)